Please try disabling https on the disk station and try again using port 5000. Disable any service that are unnecessary just for this test (Download Station, Photo Station, Web Services, File Station, etc..)
I followed your suggestion and disabled HTTPS, but I the problem was present hitting the HTTP and HTTPS versions. Strangely enough, with no other changes, I was able to log in through the external IP with no other changes, however I am still seeing the same error message when logging in from outside my personal network.
I am able to hit the DS login page, but cannot get past there, which has me believe it is not a firewall issue...